C-reactive protein antigenicity on the surface of human lymphocytes

We reported previously that C-reactive protein (CRP), when complexed to a multivalent binding specificity, can bind to a subset of lymphocytes that bear the IgG Fc receptor. Recently we showed that anti-CRP plus complement depletes natural killer function. We now show the detection of CRP on the surface (S-CRP) of a small percentage of PBL and on a minor population of phagocytic mononuclear cells. S-CRP is not detectable by direct immunofluorescence, but is readily discernible by using more sensitive techniques such as biotinylated anti-CRP with fluorescent avidin or indirect immunofluorescence with monoclonal anti-CRP. S-CRP is expressed in the absence of calcium, whereas the binding of complexed CRP is calcium dependent. S-CRP does not appear to be the exclusive binding site for CRP-CPS because anti-CRP does not block the binding of complexed CRP. Both the binding site for complexed CRP and S-CRP can be capped off; however, the kinetics of capping of these two surface molecules is different. Cells binding complexed CRP are OKT11-, whereas a significant number of PBL bearing S-CRP are OKT11+. Therefore cells bearing S-CRP appear to be a separate population from those that bind complexed CRP. These studies establish the presence of S-CRP on a subpopulation of lymphocytes, define conditions required for its detection, and speak to certain of the relationships between S-CRP and the binding site of CRP-CPS complexes.